Pneumatic die cushion equipment

ABSTRACT

A closed type cylinder unit, instead of the open-to-atmosphere type cylinder unit of conventional equipment forms part of the die cushion apparatus. Based on the principle that the die cushion capability by the closed type cylinder unit is determined by the differential pressure between the lower chamber pressure and the upper chamber pressure, the differential pressure is controlled in such manner that the lower chamber and the upper chamber communicate with each other through a check valve, which is opened when the differential pressure exceeds a preset differential pressure value. As the upper and lower chambers communicate through a shut-off valve, which hinders the excessive increase of the upper chamber pressure when the piston goes up. Further, the form of the check valve is changed for conveniently changing and adjusting the die cushion apparatus during press fabrication.

BACKGROUND OF THE INVENTION

The present invention relates to a pneumatic die cushion apparatus.

A pneumatic die cushion apparatus of conventional type is shown in FIG.7. In the figure, reference numeral 1 designates an air cylinder, whichis separated by a piston 2 into an upper chamber 1U and a lower chamber1L. The upper chamber 1U is opened to the atmospheric air through anopening 5. Reference numeral 4 designates a wear plate fixed on theupper end of a piston rod 3 and receives the cushion pin (not shown).Reference numeral 6 designates a buffer tank, and this communicates withthe lower chamber 1L through a connection pipe 7.

In a die cushion apparatus of such structure, the pressure Pu in theupper chamber is equal to the atmospheric pressure and is constantbecause the upper chamber 1U is opened to the atmospheric air.Consequently, the die cushion capability or force F is obtained from thepressure Pl in the lower chamber and the sectional area Al of the piston2: F=P·A1. However, because the pressure Pl in the lower chamber isincreased when the piston 2 moves down, i.e. when the volume of thelower chamber 1L is decreased, the die cushion capability increases. Onthe other hand, die cushion capability or force, i.e. the proper blankholding pressure (F) necessary for the press fabrication, is determinedby the quality of the material, and it is inconvenient when the forceincreases too much.

For this reason, the conventional type apparatus is provided with abuffer tank, which has a volume 5-8 times as large as the volume of thecylinder 1. This avoids an extreme increase of the die cushion force Fby enlarging the apparent or effective volume of the lower chamber 1L.

Therefore, as shown for the sake of comparison with the presentinvention in FIG. 2 by the two-dot chain line, the necessary blankholding pressure Fp can be obtained as soon as the piston 2 begins to godown from the upper limit UL if the initial pressure P1S necessary forobtaining the die cushion capability (the proper blank holding pressure)Fp is established in the lower chamber 1L and the buffer tank 6.Thereafter, the pressure gradually increases as shown by the one-dotchain line in FIG. 2. Thus, the die cushion capability when the piston 2reaches the lower limit LL is Fpe.

As described above, the die cushion apparatus of the conventional typeis constructed in such manner as to increase the effective volume of thelower chamber 1L by introducing a large-capacity buffer tank and toobtain the proper blank holding pressure at a constant level.

With the introduction of large-size presses and the automated equipmentsuch as transfer presses, and with the increasing demand for higherproduct quality and productivity, the following problems arise:

(1) The space required for the buffer tank 6 is large and not veryeconomical, and this also hinders the installation of a large-size pressand the layout of the other functional components. Particularly, itcauses significant problems in a transfer press equipped with a largenumber of dies.

(2) Much time is required for establishing the initial pressure P1S orfor an adjustment to increase the die cushion capability. This has thedrawback that, even if the other initial conditions can be met withinshort time, the press cannot be operated quickly. On the other hand, toestablish the initial pressure P1S quickly, a high-pressurelarge-capacity compressor must be installed, and this is disadvantageousdue to the expense and the space requirements.

Also, for providing an adjustment to decrease the die cushioncapability, a quick-acting large-size exhaust valve must be installedand this also causes an economic burden. Since a large quantity of highpressure air must be released to the atmosphere air or supplied for eachadjustment, this leads to an additional economic disadvantage.

(3) Further, even when the disadvantages of (1) and (2) above areaccepted, it is impossible to increase the capacity of the buffer tank 6infinitely. Therefore, the die cushion capability inevitably increasesmore or less as the piston 2 goes down.

However, the higher quality and the cost reduction are in demand thesedays, and the increase of the die cushion capability itself is no longeracceptable in many fields of the fabrication industry.

(4) In addition the materials for press fabrication have becomeincreasingly complicated and sophisticated from the viewpoints of costreduction or the quality improvement of the end products. In some cases,defective products may be produced depending upon the forms of thematerials or fabrication unless the die cushion for press fabrication ischanged adequately according to the circumstances. In other words, ifthe die cushion capability can be properly changed and adjusted duringthe press fabrication, a wide variety of products can be fabricated athigh efficiency, and extensive cost reduction along with the productionof high quality products can be actualized. However, this cannot beaccomplished by conventional pneumatic die cushion equipment.

SUMMARY OF THE INVENTION

The object of the present invention is to offer a small and compactpneumatic die cushion apparatus with simple structure, easymaneuverability and low cost, which can eliminate the large capacitybuffer tank, the large-size compressor, the quick-acting large-sizeexhaust valve, and related components, and can maintain the die cushioncapability at a constant level.

To solve these problems, the apparatus according to the presentinvention consists of a closed type cylinder unit, which is to replacethe open-to-atmosphere type cylinder of conventional equipment. The newapparatus is also based on the principle that the die cushion capabilityor force by the closed type cylinder is determined by the differentialpressure between the lower chamber pressure and the upper chamberpressure, and the differential pressure is controlled by providingcommunication between the lower and upper chambers.

In the actual detail, the lower chamber and the upper chamber having thecylinder of the piston therebetween communicate with each other througha check valve, which is opened when the differential pressure betweenthe lower chamber pressure and the upper chamber pressure, changing withthe downward movement of the piston, exceeds a preset value. Also, bothchambers communicate with each other through a shut-off valve, whichhinders an excessive increase of the upper chamber pressure when thepiston goes up.

Therefore, in the apparatus according to this invention, the lowerchamber pressure increases and the differential pressure between thelower chamber pressure and the upper chamber pressure rapidly increasesas the piston goes down from the upper limit due to the loading, and thedie cushion capability corresponding to the preset differential pressurevalue is established.

Further, when the piston goes down, the differential pressure betweenthe two chambers exceeds the preset value. Then, the check valve isopened, and the upper and the lower chambers are communicated with eachother. Consequently, the lower chamber pressure is decreased, and thedifferential pressure is decreased to less than the preset value. Atthat time, the check valve is closed.

Thereafter, the opening and the closing of the check valve is repeatedas the piston goes down, and the differential pressure between twochambers fluctuates within the narrow range of the proper blank holdingpressure. The die cushion capability is maintained at a constant leveluntil the piston reaches the lower limit.

On the other hand, when the slide goes up, the piston goes up from thelower limit to the upper limit by the differential pressure between twochambers. Then, the pressure in the upper chamber gradually increases,and when the pressures in both chambers are approximately equal to eachother, the shut-off valve is opened, and the upper and the lowerchambers are communicated with each other. As the result, the excessiveincrease of the upper chamber pressure is hindered, and the pressures inboth chambers become equal to each other.

Under such conditions, the piston is pushed upward by the difference ofthe effective area due to the presence of the piston rod (sectionalarea), and the piston goes up relatively slowly.

Because residual pressure exists within the upper chamber, the pistongoes up to the upper limit without generating a high impact force.

By establishing the proper timing to close the shut-off valve, it ispossible to extensively increase the upper limit damper effect.

The apparatus according to the present invention is of such structurethat, the upper and lower chambers of the cylinder are communicated withother through a check valve, that the check valve is opened when thedifferential pressure between two chambers reaches the presetdifferential pressure value, and the excessive increase of the upperchamber pressure is hindered by the shut-off valve when the piston goesup. Thus, the invention offers a small and compact pneumatic die cushionapparatus, which can eliminate the conventional large-size buffer tank,the high-pressure large-capacity compressor, the large-size quick-actingexhaust valve, and related components and which can maintain the diecushion capability at a constant level with a low operating cost.

Another object of this invention is to offer a pneumatic die cushionapparatus, by which it is possible to change and adjust the die cushioncapability during press fabrication.

To attain these objects, the apparatus according to the presentinvention consists of a closed type cylinder unit, which is to replacethe open-to-atmosphere type cylinder of conventional equipment. Based onthe principle that the die cushion capability by the closed typecylinder is determined by the differential pressure between the lowerchamber pressure and the upper chamber pressure, it is possibleaccording to this apparatus to adjust the die cushion capability throughthe control of differential pressure during the press fabrication bycommunicating the lower and the upper chambers with each other.

Specifically, the apparatus according to this invention is characterizedin that the lower and the upper chambers having a piston of the cylindertherebetween are communicated with each other through a first airpassage equipped with a first shut-off valve and a second air passageequipped with a second shut-off valve,

the first shut-off valve is constructed in such manner that it is openedwhen the differential pressure between the lower chamber pressure andthe upper chamber pressure exceeds the preset differential pressurevalue inputted by the differential pressure setting means,

a control member is provided to control the opening and the closing ofthe second shut-off valve when the piston goes up,

a capability memorizing means to memorize two or more capabilitydiagrams and a selection means to select one of the capability diagramsmemorized by this capability memorizing means is provided,

and the die cushion capability can be changed and adjusted during pressfabrication by changing the preset differential pressure value outputtedby the differential pressure setting means according to the capabilitydiagram selected by the selection means.

Therefore, if the preset differential pressure value outputted by thedifferential pressure setting means is constant, the lower chamberpressure increases when the piston goes down from the upper limit due tothe loading. Then, the differential pressure between the lower chamberpressure and the upper chamber pressure quickly increases, and the diecushion capability corresponding to the preset differential pressurevalue is thus established.

Further, when the piston goes down, the differential pressure betweentwo chambers exceeds the preset differential pressure value. Then, thefirst shut-off valve serving as a check valve is opened, and the lowerand the upper chambers are communicated with each other. Consequently,the lower chamber pressure is reduced, and the differential pressuregoes down to below the preset differential pressure value. At the sametime, the first shut-off valve is closed.

Thereafter, the opening and the closing of the first shut-off valve isrepeated as the piston goes down, and the differential pressure betweentwo chambers is changed within the narrow range of the tolerance of thenecessary blank holding pressure. Thus, the die cushion capability canbe substantially maintained at a constant level until the piston reachesthe lower limit.

If a capability diagram is selected by the selection means from thecapability diagrams memorized by the capability memorizing means, thepreset differential pressure value inputted to the first shutoff valveby the differential pressure setting means corresponds to the capabilitydiagram. Therefore, if the capability diagram is prepared in advance to"gradual increase", "gradual decrease", "stepwise increase or decrease",etc. for the press fabrication, the die cushion capability can beautomatically changed and adjusted during the press fabrication.

On the other hand, when the slide begins to go up, the piston goes upfrom the lower limit to the upper limit by the differential pressurebetween two chambers. Accordingly, the upper chamber pressure isgradually increased, and when the pressures on two chambers become equalto each other, the second shut-off valve is opened by the control meansand the upper and the lower chambers are communicated with each other.As the result, the excessive increase of the pressure in the upperchamber is avoided, and the pressures in two chambers are equalized.

Under such conditions, the piston is pushed upward by the effective areadifference due to the presence of the piston rod (that is, by itscross-sectional area), and the piston goes up relatively slowly.

Because residual pressure exists in the upper chamber, the pistonreaches the upper limit without generating a high impact force.

By establishing the proper timing to close the second shut-off valve, itis possible to extensively increase the effect for the upper limitdamping.

The apparatus according to this invention is constructed in such mannerthat the upper and the lower chambers of the cylinder are communicatedwith each other through the first and the second air passages, the diecushion capability is adjusted through the control of differentialpressure by the first shut-off valve, and the second shut-off valve isproperly controlled when the piston goes up. Thus, the apparatus of thisinvention can be designed in a small and compact form, and it caneliminate the large-size buffer tank, the high-pressure large-capacitycompressor, the quick-acting large-size exhaust valve, and related partsof the conventional type equipment. With low operating cost, the presentapparatus can attain the proper die cushion capability accurately andquickly.

Moreover, the differential pressure is controlled according to thecapability diagram memorized by the capability memorizing means. Thismakes it possible to change and adjust the die cushion capability duringthe press fabrication to suit the product to be fabricated, and thiscontributes to the production of a wide variety of the high qualityproducts at higher efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic view of a first embodiment of thisinvention.

FIG. 2 is a graph to explain the operation of the preferred embodimentof FIG. 1 in comparison with the operation of conventional equipment.

FIG. 3 is a general schematic view to show a second preferred embodimentof the invention.

FIG. 4 is a general schematical view to show a third preferredembodiment of the invention.

FIG. 5 is a general schematical view to show a fourth preferredembodiment.

FIG. 6 is a graph to explain the operation of the third and the fourthembodiments of FIGS. 4 and 5, respectively, in comparison with theoperation of conventional die cushion equipment.

FIG. 7 is a general schematic view of PRIOR ART die cushion equipment.

In the figures, 1 refers to a cylinder, 1U an upper chamber, 1L a lowerchamber, 2 a piston, 10 a check value, 11 a pipe, 20 a first shut-offvalve, 21 a pipe, 30 a first air passage, 31 a pipe, 40 a first shut-offvalve, 41 a main unit, 50 a second air passage, 51 a second shut-offvalve, 60 and 70 a pressure regulating valve and a control unitrespectively to form the differential pressure setting means, 71 a meansto generate the capability signal, 72 a capability memorizing means, 73a selection means, 74 a standard capability setter, 75 and 76 acontroller and a pressure setter respectively to form the control means,and 77 and 78 a controller and a pressure setter respectively to formthe initial pressure setting means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 is a general schematic view of a preferred embodiment of thisinvention, and FIG. 2 is a diagram to explain the operation of theequipment. The same number or symbol is used to designate the same orcomparable component as that of FIG. 7, which illustrates die cushionequipment of the conventional type.

As shown in FIG. 1, the die cushion apparatus of the first embodiment ofthis invention most basically includes cylinder unit (cylinder 1 andpiston 2), a check valve 10, a shut-off valve 20, and additionalcomponents enumerated above.

To the cylinder unit, the compressed air is supplied from an air source15 comprising a small compressor, a pressure regulating valve 16 and acheck valve 17. In the initial condition where the piston 2 is at theupper limit UL, the initial pressure P1S is established in a lowerchamber 1L, which is formed below the piston 2 of the cylinder 1. Thisinitial pressure P1S is set by the pressure regulating valve 16. Themovable stroke (ST) of the piston 2 is the upper limit UL and the lowerlimit LL in the cylinder 1. Reference number 1U represents the upperchamber formed above the piston 2.

Therefore, the die cushion capability of this equipment consisting of aclosed cylinder unit is determined by the differential pressure betweenthe lower chamber pressure Pl and the upper chamber pressure PU if thesectional area A2 of the piston rod 3 is neglected.

The check valve 10 is a means to communicate the lower chamber 1L withthe upper chamber 1U, and it is opened when the differential pressureΔ(=Pl-Pu) between the lower chamber pressure Pl and the upper chamberpressure Pu, changing as the piston 2 goes down, reaches the presetdifferential pressure value ΔS (e.g. 3 kgf/cm²). The check valve isinstalled in the middle of the pipe 11, connecting two chambers 1L and1U.

The check valve 10 of this embodiment is constructed in such manner thatthe die cushion capability or force corresponding to the proper blankholding pressure can be changed and adjusted.

Specifically, when the setting signal is received from a differentialpressure setter 12, the check valve 10 is opened when the differentialvalue reaches the preset value ΔS (e.g. 2.8 kgf/cm², 3.0 kgf/cm², 3.2kgf/cm², . . . . ).

In addition, a the shut-off valve 20 is installed in the middle of apipe 21, connecting the upper chamber 1U and the lower chamber 1L, andshut-off valve 20 functions to communicate the upper chamber 1U with thelower chamber 1L at an appropriate time. In other words, shut-off valve20 prevents and excessive increase of the upper chamber pressure Pu whenthe piston 2 goes up; it is closed when the piston 2 goes down, and itis opened at the appropriate time when the piston goes up.

The excessive increase of the pressure as mentioned in the presentinvention is defined as the increase of the upper chamber pressure Pu toa value higher than the lower chamber pressure Pl or as the increase ofthe upper chamber pressure Pu to a value, at which the piston 2 isunable to go up, if the sectional area A2 of the piston rod 3 is takeninto account.

Actually, the opening and closing of shut-off valve 20 are controlled bya control means 22. As shown in FIG. 2, the control means 22 of thisembodiment is constructed in such manner that the opening signal isissued to the shut-off valve 20 when the piston 2 goes up from the lowerlimit LL with the lower chamber pressure P1 reduced and the upperchamber pressure Pu increased and the pressures in two chambers P1 andPu become equal to each other (with the lower chamber pressure P1slightly higher), and that the closing signal is issued immediatelybefore the piston 2 reaches the upper limit UL.

The time to issue the opening signal can be determined by the ascendingspeed of the piston 2, and the signal may be issued at any point of thestroke (ST). It is also desirable to set the timing for the closingsignal in such manner that a better upper limit damping effect can beobtained until the piston 2 can reach the upper limit UL.

Also, such timing can be defined by the present position of the piston 2(e.g., by the position of piston rod 3) detected by a position detector23. However, the timing may be defined by detecting the differentialpressure between the lower chamber pressure P1 and the upper chamberpressure Pu or by detecting the axial angle of a crank.

The shut-off valve 20 and the control means 22 are not limited to thestructures as described above. A pilot check valve may be provided todirectly connect the upper chamber 1U with the lower chamber 1L, and thepipe 21 and the like may be omitted.

19 represents an exhaust valve to move the cushion downward.

In use, the operation of the first embodiment is as follows.

Suppose that the sectional area of the piston 2 is Al when the piston 2is at the upper limit UL. Then, the die cushion capability F isdetermined by the following equation:

    F=P1·A1-Pu·(A1-A2)

As shown in FIG. 2, the upper chamber pressure Pu is supposed to beequal to the lower chamber pressure P1 at the upper limit UL in thisembodiment, and the upward movement, i.e. the differential pressure Δ isbased on the effective area difference due to the presence of thecross-sectional area A2 of piston rod 3.

In the structure of conventional equipment as shown in FIG. 7, the upperchamber 1U is open to the atmospheric air. Accordingly, the die cushioncapability or force gradually increases as shown by the one-dot chainline of FIG. 2 and reaches Fpe at the lower limit LL. Thus, thepredetermined die cushion capability Fp cannot be maintained at constantlevel. The blank holding pressure becomes too high by 20-25% at thelower limit LL.

If the differential pressure ΔS is set on the differential pressuresetter 12 in advance to communicate the lower chamber 1L with the upperchamber 1U in this embodiment, the predetermined die cushion capabilityor force can be quickly obtained because the lower chamber pressure P1is increased and the differential pressure Δ from the upper chamberpressure Pu is increased when the piston moves downward. When thedifferential pressure Δ becomes equal to the preset differentialpressure value ΔS, the check valve 10 is opened, connecting two chambers1L and 1U with each other. As a result, the lower chamber pressure P1decreases and the upper chamber pressure Pu increases. Then, thedifferential pressure Δ becomes smaller than the preset differentialpressure ΔS, and the check valve 10 is closed again. Thus, the checkvalve 10 repeatedly undergoes the opening and closing operationaccording to the differential pressure Δ until the piston 2 reaches thelower limit LL.

Therefore, the die cushion capability Fl is controlled within a band Bof constant width even though there is slight change of differentialpressure (as shown by the solid line zigzag in FIG. 2) according to theopening and closing operation of the check valve 10.

The width of the band B is defined as the allowable range of the properblank holding pressure. The width of the band B can be selected bysetting the bore diameter, flow resistance, and the like of the checkvalve 10 and the pipe 11, relative to the volume of the cylinder 1, toappropriate values.

Also, the die cushion capability can be varied according to the presetdifferential pressure value ΔS of the differential pressure setter 12.When it is set to the capability F2 as shown in FIG. 2, the presetdifferential pressure value ΔS should be set to a value lower than thatof the capability F1.

On the other hand, the upward movement of the piston 2 from the lowerlimit LL depends upon the differential pressure Δ between the pressuresP1 and Pu of two chambers and matches the upward movement of the slidein the initial stage. Then, it is performed smoothly in non-loadedstatus. As the lower chamber pressure P1 decreases and the upper chamberpressure Pu increases, the differential pressure Δ rapidly decreases.

Then, shut-off valve 20 is opened by the signal from the control means22, and the two chambers 1U and 1L are connected with each other.Accordingly, the pressures P1 and Pu of two chambers become equal toeach other, while the piston 2 moves further upward by the pushing forcegenerated by the effective area difference due to the presence of thesectional area A2 of the piston rod 3.

This piston may be left to go up to the upper limit UL, whereas theshut-off valve 20 is closed again in this embodiment immediately beforethe upper limit UL. Therefore, the upper chamber pressure Pu becomesslightly higher and rapidly reduces the pushing force of the piston 2.Thus, the better damping effect can be obtained at the upper limit UL.

It is also effective that the shut-off valve 20 is opened and closedagain momentarily when the piston 2 closely approaches the upper limitUL.

In the first embodiment, the lower chamber 1L and the upper chamber lUare communicated with each other, and the communication is opened by thecheck valve 10 when the differential pressure between the pressures P1and Pu of two chambers reaches the preset differential pressure ΔS.Accordingly, the differential pressure between the upper chamberpressure Pu and the lower pressure P1 can be kept at constant level, andthe die cushion capability can be maintained at a constant level duringthe press fabrication.

Because this embodiment is based on a closed construction where theupper chamber lU and the lower chamber 1L are connected with each otherthrough the check valve 10, the pipe 11, the shut-off valve 20, and thepipe 21, and where the air in the lower chamber 1L is released into theupper chamber 1U when the piston 2 goes down. Hence, there is no need toinstall the extra-large buffer tank 6 as in conventional equipment, andthis provides conveniences and advantages in terms of operationaleconomy, space requirements, and air consumption.

Because the die cushion capability can be adjusted by the crackingpressure (differential pressure) of the check valve 10, there is no needto install a high-pressure large-capacity compressor or large-sizeexhaust valve as in conventional equipment. This contributes to thecompact design of the equipment and quick adjustment operation.Therefore, the production efficiency of the press can be raised with ashorter waiting time. More economic operation can be achieved becausethere is no need to release a large quantity of compressed air into theatmosphere.

Further, the upper chamber pressure Pu can be left unequal near theupper limit UL of the piston 2, i.e. residual pressure may be providedby setting the proper time to close the shut-off valve 20. This ishelpful to extensively decrease the impact force at the upper limit UL.

Because the cracking pressure of the check valve 11 can be easily setand changed by the differential pressure setter 12, the necessary blankholding pressure can be quickly and accurately obtained. Moreover, theinconveniences such as bending of or other damage to the flange can beavoided because the blank holding pressure remains constant regardlessof the downward movement of the piston 2. This contributes to the stableproduction of high-quality products.

Further, because the shut-off valve 20 is so constructed that itsopening and closing are controlled properly by the control means 22, thereturn speed of the piston 2 to the upper limit UL, the upper limitdamping effect, etc. can be set to the values as desired.

Embodiment 2

In contrast to the embodiment of FIG. 1, in which the cracking pressurecan be set and changed by the differential pressure setter 12 equippedwith the check valve 10, the second preferred embodiment of FIG. 3 isconstructed in such manner that the check valves 10 (10A, 10B and 10C)are installed in parallel as shown in FIG. 3, and that one check valvecan be selected by the selection means 14 through changeover of thechangeover switches 13 (13A, 13B and 13C) consisting of thecorresponding electromagnetic valves. The other details are the same asthe embodiment of FIG. 1, and the explanation is omitted.

Therefore, it is possible by this second embodiment to obtain the sameoperation and effect as the embodiment of FIG. 1, to set the die cushioncapability more accurately because the cracking pressure of the checkvalve 10 is constant, and to perform the setting changeover more quicklybecause the changeover is carried out by simply opening or closing thechangeover valve.

The embodiments described above consist of a complete closed typecylinder unit (cylinder 1 and piston 2), whereas an opening valve 25 maybe provided to open the upper chamber lU to atmospheric air at theproper time and temporarily as shown in FIG. 3 by two-dot chain line.For example, if it is opened by the control means 22 only for the momentwhen the piston 2 goes down from the upper limit UL, it is possible toperform the startup of the die cushion capability [F, (F2)] morequickly. Thus, it is possible to obtain and to promote the better effectand operation of the present invention.

Of course, in the special-purpose press with no need to change andadjust the die cushion capability, it is not necessary to change thecheck valve 10 to the cracking pressure variable type, and thisinvention is also applicable to such a case.

Embodiment 3

Compared with the embodiments of FIGS. 1 and 3 which have constant diecushion capability during press fabrication, the die cushion capabilityis changeable and adjustable during press fabrication in the thirdembodiment of FIG. 4.

As shown in FIG. 4, this die cushion apparatus comprises a cylinder unitconsisting of a cylinder 1, a piston 2, an initial pressure settingmeans, a differential pressure setting means, a capability memorizingmeans 72, a selection means 73 to control the first air passage 30 and asecond air passage 50, and a control means to control the opening andthe closing of the second air passage 50.

The cylinder unit consists of a cylinder 1, a piston 2, a piston rod 3,etc., and it has basically the same structure as the conventionalapparatus of FIG. 7.

However, because one of the objects of the present invention is toeliminate the buffer tank 6 of conventional equipment, the conventionalconnecting pipe 7 is not installed in the lower chamber, and the upperchamber 1U is designed as a closed type without a conventional opening5.

The initial pressure setting means is to set the pressure in the lowerchamber lL under the initial condition (with the piston 2 at upper limitUL), and it comprises a pressure regulating valve 36 of pressurevariable type and a check valve 37 installed on the pipe 38 connectingan air source 15 consisting of a small compressor with the lower chamber1L and a controller 77 to regulate the pressure regulating valve 36according to the setting value of the pressure setter 78.

The first air passage 30 is to connect the lower chamber lL with theupper chamber lU of the cylinder 1 when the piston 2 goes down, and itconsists of a pipe 31 connecting the two chambers lL and lU and of afirst shut-off valve 40 installed in the middle of this pipe 31 andserving as a check valve. The cracking pressure as the check valvefunction is controlled by the preset differential pressure valueinputted from the differential pressure setting means (60 and 70).

It opens when the differential pressure between the lower chamberpressure P1 and the upper chamber pressure Pu exceeds the presetdifferential pressure value.

Here, the first shut-off valve 40 consists of a hollow cylindrical mainunit 41 and a cylindrical valve disc 46 slidably inserted in the mainunit 41, and it is closed at all times.

At the front end of the main unit 41, an inlet 42I and an outlet 420 areprovided to connect with the pipe 31, and an inlet 43I is furnished onthe rear end to apply the upper chamber pressure Pu through the pipe 31.

On the other hand, a spring is mounted in the hollow portion 47H at therear end of the valve disc 46, and a valve unit 47V is provided on thefront end to close the inlet 42I.

Consequently, when the lower chamber pressure P1 is increased, the valvedisc 46 is moved rightward in FIG. 4 against the resilient force of thespring 49, and the lower chamber lL and the upper chamber 1U of thecylinder 1 are connected with each other through the inlet 42I and theoutlet 420.

Also, when the air is released from the lower chamber lL, the lowerchamber pressure P1 is reduced, and the valve 40 is closed again by theresilient force of the spring 49.

In this case, the cracking pressure serving as a check valve to thefirst shut-off valve 40 is determined primarily by the resilient forceof the spring 49.

In the meantime, the first shut-off valve 40 of this invention ischaracterized in that the cracking pressure is variable. For thisreason, compressed air is supplied from the inlet 43C furnished in themiddle of the main unit 41. The compressed air serving as a controlsignal is to increase the cracking pressure in addition to the resilientforce of the spring 49. In other words, minimum cracking pressure isdetermined by the spring 49, and the higher cracking pressure isdetermined by the pressure of the air supplied to the inlet 43C. Theoutlet 420 and the inlet 43I are communicated with each other to keepthe balance. Also, 430 is opened to the atmospheric air.

Here, the differential pressure setting means is a means to set thecracking pressure of the first shut-off valve 40 and is to set thepressure of the air supplied to the inlet 43C of the first shut-offvalve 40. Specifically, it includes of a pressure regulating valve 60serving as an electropneumatic converter installed on the pipe 61, whichconnects the inlet 43C of the valve 40 with the air source 15, and acontrol unit 70. The control unit 70 and the associated elements areaccommodated in the control panel 80 together with the other controllers(75) and the like.

The control unit 70 in this third embodiment of FIG. 4 uses the lowerchamber pressure P1 from the pressure detector 32, the upper chamberpressure Pu from the pressure detector 53 and the crank axial angle fromthe angle detector 85 as input data. It issues the electric signal tocontrol the pressure regulating valve 60 so that the differentialpressure obtained by the comparative computation of the lower chamberpressure P1 and the upper chamber pressure Pu becomes equal to thedifferential pressure corresponding to the capability signal. In otherwords, it is to control the cracking pressure of the first shut-offvalve 40 by a closed loop.

This capability signal is outputted from the capability signalgenerating means 71. The capability signal generating means 71 is todetermine the die cushion capability to obtain the necessary blankholding pressure during press fabrication in relation to the stroke ofthe piston 2, and a capability memorizing means 72 to memorize two ormore capability diagrams, i.e. the die cushion capability - pistonstroke curve is included in it. The capability diagram is as shown bythe curves (2)-(5) in FIG. 6, and the capability is changed during thechange of the piston stroke, i.e. during press fabrication. For widerapplication, the curve (1) is also memorized, in which the capability isat constant level as in the cases of the embodiments 1 and 2.

The selection of the curve from the capability diagram is carried out bythe selection changeover means 73.

Further, the capability signal generating means 71 in this thirdembodiment is designed in such manner that the capability signal for thequick rising up to the standard capability F as set by the standardcapability setter 74 until the piston stroke ST1 as shown in FIG. 6 isreached.

Of course, the diagram specifying from the upper limit UL to the lowerlimit LL and from the lower limit LL to the upper limit UL may bememorized by the capability signal generating means 71 and thecapability signal generating means 71 may issue the capability signalcorresponding to the diagram selected by the selection means 73 to thecontrol unit 70. In this case, the standard capability setter 74 can beomitted. (See the embodiment 4.)

The piston stroke is to be determined by the axial angle of the crankinputted from the angle detector 85. Because the capability memorizingmeans 72 consists of reloadable ROM (read-only memory) the capabilitydiagram can be changed, added or deleted as appropriate.

Next, the second air passage 50 is to connect the upper chamber lU withthe lower chamber lL at appropriate time during the upward movement ofthe piston 2, and it is composed of a pipe 52 to connect two chambers lUand lL and of a second shut-off valve 51 consisting of anelectromagnetic valve.

The control means to control the opening and the closing of the secondshut-off valve 51 consists of a pressure setter 76 and a controller 75.The controller 75 issues the signal to excite the solenoid when theupper chamber pressure Pu detected by the pressure detector 53 is equalto the setting value of the setter 76. When the solenoid is excited, thesecond shut-off valve 51 is opened. Also, the controller 75 turns offthe signal when the piston 2 closely approaches the upper limit UL andcloses the second shut-off valve 51 again.

The differential pressure setter may be provided instead of the pressuresetter 76, and the lower chamber pressure P1 may be inputted to thecontroller 75 so that the opening and the closing of the second shut-offvalve 51 can be controlled according to the differential pressurebetween the pressures P1 and Pu of two chambers.

19 represents an exhaust valve to move the cushion downward, and 37 acheck valve.

In the third embodiment with the structure as described above, theoperation is performed as follows:

First, the initial pressure in the lower chamber 1L is set by thepressure setter 78. Next, the standard capacity F to be determined up tothe piston stroke ST1 is set by the standard capability setter 74, andthe curve, in which the capability after the stroke ST1 matches theforms of the products, is selected by the selection means 73. (e.g. thecurve (5) of FIG. 6) Also, the upper chamber pressure Pu to close thesecond shut-off valve 51 is set by the pressure setter 76.

In FIG. 6, the curves (2)-(5) are more simplified than the curve (1)indicating the average capability only), and the process during theupward movement of the piston is not shown because it is easilyimaginable from a consideration of the curve (1).

The die cushion capability F is determined by the following equation:

    F=P1·A-Pu·(A1-A2)

Because the upper chamber lU is open to the atmospheric air in theconventional structure as shown in FIG. 7, the predetermined capabilityFa is determined as soon as the piston rod 3 moves down. Then, the diecushion capability gradually increases up to Fae at the lower limit LLas shown by the one-dot chain line of FIG. 6. Because the volume of thebuffer tank 6 is 5-8 times more than that of the cylinder, it is notpossible to maintain the die cushion capability Fa at a constant value.At the lower limit LL, the blank holding pressure becomes excessive by20-25%.

When the piston 2 moves down from the upper limit UL in the thirdembodiment, the lower chamber pressure P1 increases, and thedifferential pressure from the upper chamber pressure Pu increases.Thus, the standard capability F can be generated quickly. The higher theinitial pressure in the lower chamber 1L is, the quicker the startup is.

When the piston 2 tends to go down after the standard capability F isestablished, the control unit 70 including the differential pressuresetting means controls the pressure regulating valve 60 in such a mannerthat the differential pressure obtained from the input given by thepressure detectors 32 and 53 becomes equal to the differential pressurevalue corresponding to the capability signal issued from the capabilitysignal generating means 71. This determines the cracking pressure, andthe first shut-off valve 40 is opened by this cracking pressure. Then,the lower chamber lL is connected with the upper chamber lU. The lowerchamber pressure P1 decreases and the first shut-off valve 40 closesagain.

Therefore, the differential pressure changes, repeating the slightfluctuation within the allowable range of the necessary blank holdingpressure. The standard capability F can be substantially maintained upto the stroke ST1.

If the curve (1) of FIG. 6 is selected by the selection means 73, thecapability signal generating means 71 reads the curve (1) from thediagrams memorized in the capability diagram memorizing means 72 andissues this to the control unit 70. In this case, the first shut-offvalve 40 is controlled by the differential pressure setting means (60and 70) in such manner that the capability F can be maintained at theconstant level until the piston 2 reaches the lower limit LL.

If the curve (5) is selected, for example, the control unit 70 controlsthe pressure regulating valve 60 to change the die cushion according tothe curve (5) based on the input from the capability signal generatingmeans 71, the input from both pressure detectors 32 and 53, and theinput from the angle detector 33. Thus, the opening and the closing ofthe first shut-off valve 40 are controlled. The die cushion capabilityis decreased stepwise after the piston stroke passed ST1 during pressfabrication, and it is maintained at constant level until the pistonreaches the lower limit LL.

In case of the curve (4), it gradually decreases, whereas in case of thecurve (2), it gradually increases.

On the other hand, the upward movement of the piston 2 from the lowerlimit LL matches initially the upward movement of the slide according tothe differential pressure between the pressures P1 and Pu in twochambers, and the piston moves smoothly upward thereafter in non-loadedstatus. Because the lower chamber pressure P1 decreases and the upperchamber pressure Pu increases, the differential pressure rapidlydecreases.

When the upper chamber pressure Pu exceeds the preset value determinedby the pressure setter 76, the second shut-off valve 51 is opened by thesignal of the controller 75, and two chambers lU and lL are connectedwith each other. Accordingly, the pressures P1 and Pu in two chambersbecome equal to each other, whereas the piston 2 moves further upward bythe pushing force generated by the effective area difference due to thepresence of the sectional area A2 of the piston rod 3.

The piston may be left to reach the upper limit UL, while, in thisembodiment, the second shut-off valve 51 is closed again immediatelybefore the upper limit UL. Because the upper chamber pressure Puslightly increases and reduces the pushing force of the piston 2, thedamping effect at the upper limit UL can be extensively increased.

It is also effective to open and close the shut-off valve 51 againmomentarily when the piston 2 closely approaches the upper limit UL.

In the embodiment 3, the first and second air passages 30 and 50 areprovided to connect the upper chamber 1U with the lower chamber lL ofthe cylinder 1. When the piston goes down, the opening and the closingof the first air passage 30 (the first shut-off valve 40) are adjustedto control the differential pressure between the lower chamber pressureP1 and the upper chamber pressure Pu. When the piston goes up, theopening and the closing of the second air passage 50 (the secondshut-off valve 51) are adjusted to hinder the excessive increase of theupper chamber pressure Pu. Consequently, there is no need to installextra-large buffer tank (6), and it provides many advantages in terms ofoperation economy, space requirements and air consumption.

Also, the first shut-off valve 40 having the function of a check valveis included in the first air passage 30, and the setting of the crackingpressure is changed by the differential pressure setting means (60 and70) in the first shut-off valve 40. Moreover, the differential pressuresetting means (60 and 70) controls the first shut-off valve 40 accordingto the capability signal issued from the capability signal generatingmeans 71 specified by the selection means 73 and the capabilitymemorizing means 72. Thus, the die cushion capability can be changedduring the press fabrication, and a wide variety of the products can beproduced efficiently. Also, the material cost can be reduced, and thereis no restriction on the forms of the material to be used. The start-upand the stopping of the operation can be quickly adjusted.

The cylinder units (1, 2) are designed as closed type units, and the diecushion capability is determined by connecting or separating twochambers 1U and 1L through the control of the first air passage 30 andthe second air passage 50. This contributes to the elimination of thehigh-pressure large-capacity compressor or the quick-acting large-sizeexhaust valve as seen in conventional equipment, and a compact andeconomical apparatus can be obtained. To adjust the die cushioncapability, only the setting of the cracking pressure of the firstshut-off valve 40 should be changed. This means quick and accurateoperation as well as a shorter waiting time and higher productionefficiency of the press. Moreover, the quantity of the air released forthe adjustment of die cushion capability is very low, in principle, andit is very advantageous for economical operation.

Further, the upper chamber pressure Pu can be left unused when thepiston approaches the upper limit UL; i.e., the differential pressurecan be reduced by choosing the closing time of the second shut-off valve51 appropriately. This is helpful to extensively decrease the impactforce at the upper limit UL.

Embodiment 4

The fourth embodiment is shown in FIG. 5.

The fourth embodiment has more simplified components and facilities thanthe third embodiment.

Specifically, when the forms and the characteristics of the cylinderunit (1, 2), the first shut-off valve 40 and the second shut-off valve51 are determined, the lower chamber pressure P1, the upper chamberpressure Pu and the differential pressure between the pressures P1 andPu of two chambers during the upward and the downward movement of thepiston 2 as well as the relation between the upward and the downwardmovement of the slide and the necessary blank holding pressure can bemade clear if the type of the press and the products to be fabricatedare specified. Therefore, the differential pressure is controlled by thefirst shut-off valve 40, whereas the pressures in two chambers and thedifferential pressure are not directly detected and these aresubstituted for by the axial angle of the crank.

Therefore, the pressure detectors 32 and 53 in the third embodiment ofFIG. 4 are not included.

Also, the standard capability setter 74 is not provided, and the diecushion capability for all strokes of the piston 2 from the upper limitUL to the lower limit LL is memorized by the capability diagrammemorizing means 72. Thus, the control unit 70 controls the pressureregulating valve 60 by specifying the capability signal, inputted fromthe capability signal generating means 71, by the crank axial angle fromthe angle detector 85, and the cracking pressure of the first shut-offvalve 40 is set. That is, the embodiment 3 forms a closed loop, whilethe embodiment 4 forms an open loop with one curve read from thecapability diagram memorizing means 72 as the preset value.

Further, the control means to control the opening and the closing of thesecond shut-off valve 51 includes the controller 75, which is a programsequencer. Controller 75 controls the opening and the closing of thesecond shut-off valve 51 in relation to the crank axial angle accordingto a predetermined procedure during the upward movement of the piston 2.However, the opening and the closing are controlled in their timingsimilar to control in the third embodiment. The opening and closingprocedure as well as the timing can be changed.

It is also possible in the case of the fourth embodiment to obtain thesame effect as the third embodiment by using the crank axial angle as aninput, (i.e. the effects such as the elimination of large size buffertank, high-pressure large-capacity compressor, quick-acting large-sizeexhaust valve, and the adjustment and alteration of the die cushioncapability during press fabrication).

Moreover, compared with the third embodiment, this fourth embodiment canprovide a more simplified structure and lower cost by omitting twopressure detectors (32 and 53) and by simplifying the control unit 70.

Because each operation is determined not as the differential pressurebetween the pressures P1 and Pu in two chambers but is indirectlydetermined by the crank axial angle, the automatic adjustment of the diecushion capability can be freely, easily and simply performed duringpress fabrication if the capability diagram to be memorized by thecapability memorizing means 72 is clearly defined.

In the third and fourth embodiments, the control unit 70, the capabilitysignal generating means 71, the capability memorizing means 72, and thecontrollers 75 and 77 are furnished separately, whereas these means maybe organically and integrally incorporated by a computer consisting of aCPU (central processing unit), with the necessary RAM (random-accessmemory), ROM (read-only memory), and the like.

What is claimed is:
 1. A pneumatic die cushion apparatus, comprising:afirst and a second shut-off valve; a cylinder having a piston movableupwardly and downwardly therein, and upper and lower chambers in saidcylinder respectively above and below said piston, and said upper andlower chambers communicating with each other by a first air passagethrough said first shut-off valve and communicating with each other by asecond air passage through said second shut-off valve; a differentialpressure setting means for establishing and outputting a presetdifferential pressure value established by the difference between thepressure in the upper chamber and the pressure in the lower chamber;means for operating said first shut-off valve when the differencebetween the lower chamber pressure and the upper chamber pressureexceeds the present differential pressure value inputted from thedifferential pressure setting means; a control means for controlling theoperating of opening and closing of said second shut-off valve at apredetermined time when said piston goes up; and a capability diagrammemorizing means for memorizing at least two capability diagrams andincluding a selection means for selecting one of said at least twocapability digrams memorized by said capability diagram memorizingmeans, wherein the preset differential pressure value issued from thedifferential pressure setting means is changed according to the one ofsaid at least two capability diagrams selected by the selection means tochange and adjust a die cushion capability during press operation.
 2. Apneumatic die cushion apparatus as set forth in claim 1, wherein saidfirst shut-off valve includes a hollow main unit having an inletconnected to the lower chamber of the cylinder and an outlet connectedto the upper chamber, and said first shut-off valve includes a valvedisc slidably engaged on said hollow main unit, and spring means forcausing said valve disc to normally close said inlet and to establishthe minimum cracking pressure, and a pneumatic means for supplying airpressure to determine and to vary a higher cracking pressure.
 3. Apneumatic die cushion apparatus as set forth in claim 2, furthercomprising a pressure setter for setting a value and a pressure detectorfor detecting the upper chamber pressure, and wherein said control meansopens said second shut-off valve when the value set by the pressuresetter becomes equal to the upper chamber pressure of said cylinderdetected by the pressure detector.
 4. A pneumatic die cushion apparatusas set forth in claim 2, wherein said control means includes said secondshut-off valve connecting said upper chamber with said lower chamber,and a program sequencer.
 5. A pneumatic die cushion apparatus as setforth in claim 4, wherein said differential pressure setting meansincludes a pressure regulating valve provided between said pneumaticmeans and said first shut-off valve, and a control unit is provided toissue an electric signal for controlling said pressure regulating valve.6. A pneumatic die cushion apparatus as set forth in claim 5, furthercomprising an angle detector for detecting a crank angle correspondingto a stroke of said piston and generating an electrical signalcorresponding thereto, and wherein said control unit includes saidcapability diagram memorizing means to memorize a die cushion capabilitydefined for all strokes of said piston and issues the electric signalfor control of the output signals from the capability signal generatingmeans to the pressure regulating valve, the electric signalcorresponding to the stroke for the crank angle detected by the angledetector.
 7. A pneumatic die cushion apparatus as set forth in claim 5,further comprising a capability signal generating means for inputting adie cushion capability to said control unit, and wherein said controlunit issues the electric signal for causing the difference in pressurebetween the lower chamber pressure and the upper chamber pressure ofsaid cylinder to be equalized to the differential pressure correspondingto the die cushion capability inputted by said capability signalgenerating means.
 8. A pneumatic die cushion apparatus as set forth inclaim 7, wherein said capability signal generating means includes saidcapability diagram memorizing means and issues the capability signal asa function of the capability diagram selected by said selection means.9. A pneumatic die cushion apparatus as set forth in claim 8, furthercomprising a standard capability setter for setting a standardcapability, and wherein said capability signal generating meanscomprises means for generating the capability signal for rapid startupup to the standard capability as set by said standard capability setteruntil said piston reaches a predetermined stroke.
 10. A pneumatic diecushion apparatus, comprising;a cylinder having a piston therein, saidpiston being movable upwardly and downwardly from an upper limit to alower limit, and said piston defining upper and lower chambers; a checkvalve fluidly connecting said upper and lower chambers; means foroperating said check valve when a differential pressure between thepressure in the lower chamber and the pressure in the upper chamber,which changes with the downward movement of the piston, exceeds apredetermined differential pressure value, and for controlling thedifferential pressure from a time when the differential pressure exceedssaid predetermined differential pressure value until said piston reachessaid lower limit, during a latter portion of the downward movement ofsaid piston; and shut-off valve means for providing fluid communicationbetween said upper and lower chambers to establish a predeterminedpressure in said upper chamber and to hinder an excessive increase ofpressure in said upper chamber when said piston moves upwardly.
 11. Apneumatic die cushion apparatus as set forth in claim 10, wherein saidmeans for operating said check valve maintains the differential pressuresubstantially constant during the latter portion of the downwardmovement of said piston.
 12. A pneumatic die cushion apparatus as setforth in claim 10, wherein said means for operating said check valveincludes means for alternately opening and closing said check valve formaintaining the differential pressure substantially constant during thelatter portion of the downward movement of the piston.
 13. A pneumaticdie cushion for cushioning a downward force exerted by a press,comprising:a hollow cylinder having an upper end and a lower end, theupper end of the cylinder having an aperture; a piston in the cylinder,the piston dividing the interior of the cylinder into an upper chamberand a lower chamber, the piston being movable between an upper limitadjacent the upper end of the cylinder and a lower limit adjacent thelower end of the cylinder; a piston rod connected to the piston andextending through the aperture n the upper end of the cylinder, thepiston rod receiving the downward force exerted by the press; firstmeans for exerting an approximately constant retarding force on thepiston rod as the downward force exerted on the piston rod moves thepiston from a position adjacent the upper limit to a position adjacentthe lower limit, the first means communicating between the upper andlower chambers and including a check valve, and means for repeatedlyopening the check valve when the pressure in the lower chamber exceedsthe pressure in the upper chamber by a predetermined differentialpressure value; and second means for limiting the pressure in the upperchamber as the piston moves from the lower limit to the upper limit, thesecond means communicating between the upper and lower chambers andincluding a shut-off valve, and means for opening the shut-off valvewhen the piston has moved from the lower limit to a predeterminedposition between the upper and lower limits.
 14. A pneumatic die cushionapparatus, comprising:a check valve; a cylinder having a piston movableupwardly and downwardly therein, and upper and lower chambers in thecylinder respectively above and below said piston, and said upper andlower chambers communicating with each other through said check valve;means for operating said check valve during a downward movement of saidpiston when the difference between the pressure in said lower chamberand the pressure in said upper chamber, which changes with the downwardmovement of the piston, exceeds a preset differential pressure value,said means for operating said check valve comprising a differentialpressure setter, said check valve being opened or closed by a settingsignal of the differential pressure setter, the differential pressuresetter including means for changing the preset differential pressurevalue; shut-off valve providing communication between said upper andlower chambers; and means for operating said shut-off valve to hinder anexcessive increase of the pressure in said upper chamber when the pistongoes up.
 15. A pneumatic die cushion apparatus, comprising:a checkvalve; a cylinder having a piston movable upwardly and downwardlytherein, and upper and lower chambers in the cylinder respectively aboveand below said piston, and said upper and lower chambers communicatingwith each other through said check valve; means for operating said checkvalve during a downward movement of said piston when the differencebetween the pressure in said lower chamber and the pressure in saidupper chamber, which changes with the downward movement of the piston,exceeds a preset differential pressure value; a shut-off valve providingcommunication between said upper and lower chambers; and means foroperating said shut-off valve to hinder an excessive increase of thepressure in said upper chamber when the piston goes up, said means foroperating said shut-off valve comprising control means for generating anopening signal and a closing signal, said shut-off valve being opened bythe opening signal of said control means and closed by the closingsignal of said control means.
 16. A pneumatic die cushion apparatus asset forth in claim 15, wherein said control means issues the opening orthe closing signal when one of said piston and said piston rod comes toa predetermined position.
 17. A pneumatic die cushion apparatus,comprising:a check valve; a cylinder having a piston movable upwardlyand downwardly therein, and upper and lower chambers in the cylinderrespectively above and below said piston, and said upper and lowerchambers communicating with each other through said check valve; meansfor operating said check valve during a downward movement of said pistonwhen the difference between the pressure in said lower chamber and thepressure in said upper chamber, which changes with the downward movementof the piston, exceeds a preset differential pressure value; a shut-offvalve providing communication between said upper and lower chamber; andmeans for operating said shut-off valve to hinder an excessive increaseof the pressure in said upper chamber when the piston goes up, saidmeans for operating said shut-off valve comprising a control means forcontrolling said shut-off valve, said shut-off valve and said controlmeans being integrally incorporated in a pilot check valve, and saidpilot check valve directly communicates said upper chamber with saidlower chamber of said cylinder.
 18. A pneumatic die cushion apparatus,comprising:at least two check valves, each having a different operatingpressure; a cylinder having a piston movable upwardly and downwardlytherein, and upper and lower chambers in the cylinder respectively aboveand below said piston, and said upper and lower chambers communicatingwith each other through said at least two check valves; means forselecting one of said at least two check valves and operating theselected check valve during a downward movement of said piston when thedifference between the pressure in said lower chamber and the pressurein said upper chamber, which changes with the downward movement of thepiston, exceeds the operating pressure of the selected check valve; ashut-off valve providing communication between said upper and lowerchambers; and means for operating said shut-off valve to hinder anexcessive increase of the pressure in said upper chamber when the pistongoes up.